This invention relates generally to noise-canceling microphones and related devices. More particularly, this invention relates to a bi-directional noise control device for use with boom mounted noise-canceling microphones in environments having random noise.
Microphone units typically operate in environments where unwanted noise is present. For example, a person listening to a receptionist using a telephone headset with a boom mounted microphone may be distracted from the receptionist's voice by sounds emanating from machinery, traffic, appliances, or other ambient sounds, if the receptionist is talking into a headset without a noise-canceling microphone.
Many noise-canceling microphone element designs employ front and rear sound ports which allow sound to enter both front and rear and impinge upon the diaphragm simultaneously in opposite directions resulting in little or no signal being generated by the microphone. This technique is applied in a wide variety of cardioid microphones as well as telephone handset transmitters and headsets. Some employ acoustic tuning to the rear port to make it more frequency responsive.
Noise-canceling microphones depend upon two factors for their operation. The first factor is the polar pattern of the microphone (usually bi-directional) and the assumption that the noise to be reduced is not on the maximum sensitivity axis of the microphone. The second factor is the different responses of the bi-directional microphone for a sound source close to the microphone (i.e., entering the front sound port) and a sound source at a relative distance from the microphone (i.e., entering the front and rear sound port).
When the sound source is close to the front sound port of the microphone, the sound pressure will be several times greater at the front than at the rear. Since the microphone responds to the difference of sound pressure at the two entries, close talking will provide a substantially higher sensitivity than a remote sound, where the sound pressure is equal in magnitude at the two entries.
Because of construction restraints inherent in front and rear sound port microphone design, one port of the microphone is always more sensitive. This results from the need to provide a supporting structure for the diaphragm and the resulting impedance that structure presents to sound entering the rear sound port microphone element. In common practice, the more sensitive port is faced forward to capture the desired sound while the less sensitive port is utilized for capturing and nulling the undesired background noises.
If the front and back sensitivities of the element were equal, then theoretically 100% noise rejection would be possible whenever noise of equal pressure enters both entrances of the microphone. In practice however, only 10-20 dB noise reduction is possible using the currently available microphone elements and this is only for frequencies below about 3 KHz.
Frequency response is another factor that differentiates noise-canceling microphones. Frequency response is essentially flat in the near field (i.e., for a sound source close to the front sound port on the talker's voice side of the microphone) over the audio band. In the far field (i.e., for a remote sound source), the frequency response increases with frequency until the pressures at the front and rear ports of the unit are 180 degrees out of phase at which point resonance occurs. At some frequency, the microphone becomes more sensitive to axial far field sounds than axial near field sounds. This crossover frequency will occur at a higher frequency for a microphone with a shorter port separation than a microphone with a longer port separation.
Several devices, both electrical and mechanical, used for noise-cancellation exist but have potential drawbacks such as the need for preprocessing, effects of reflections, calibration difficulties, cost, and operating environment. For example, in environments in which human speech is the ambient noise, signal processing techniques such as filtering can not effectively be used because the ambient human speech is at the same frequency as the desired speaker's voice and because the ambient noise is non-constant or non-periodic.